Water-absorbent resin particles have been used in hygienic materials such as disposable diapers and sanitary articles, agricultural materials such as water-retaining materials and soil conditioners, and industrial materials such as water blocking materials for cables and dew-catchers. In addition to these applications, the fields in which water-absorbent resin particles are applied have further expanded in recent years to include animal waste treatment materials such as pet sheets and toilet formulations for dogs or cats, portable toilets, air fresheners, absorbent drip sheets for meats and formulations for moisturizing cosmetics. Examples of the performance required by water-absorbent resin particles used in such applications include high water-absorption capacity, superior water-absorption rate and a suitable particle size corresponding to the application.
Among these applications, a comparatively large amount of body fluid and the like is expected to be discharged with considerable force onto water-absorbent resin particles used in applications which are special hygienic materials such as adult diapers, incontinence pads, toilet training pants and heavy day sanitary napkins, water blocking materials for cables, pet sheets, portable toilets and the like. Consequently, in above applications, emphasis has been placed on improvement of absorption capacity and absorption rate. Among these, although it is possible to accommodate absorption capacity by, for example, adjusting the amount of water-absorbent resin particles used, the absorption rate demonstrates a strong tendency to depend on unique properties of the water-absorbent resin particles. Consequently, various studies have been conducted thus far to achieve a superior water-absorption rate for water-absorbent resin particles.
For example, with respect to aqueous polymerization, a method has been disclosed for crosslinking the vicinity of the surface of a porous resin obtained in the presence of a foaming agent (see Patent Literature 1).
In addition, with respect to reversed-phase suspension polymerization, a method is disclosed in which an acrylic acid/acrylate aqueous solution is suspended in an alicyclic or aliphatic hydrocarbon solvent in the presence of a surfactant having an HLB value of 8 to 12 followed by carrying out reversed-phase suspension polymerization on the acrylic acid/acrylate (see Patent Literature 2), while another method is disclosed in which a water-soluble ethylenically unsaturated monomer is polymerized in the presence of a water-absorbent resin having a different water-absorption rate (see Patent Literature 3).